State-of-the-art wavelength division multiplexed (WDM) optical fiber transmission systems employ distributed Raman amplification (DRA) in addition to discrete amplifiers. DRA partly compensates fiber losses along the transmission fiber and thus allows increasing the distance between discrete amplifiers. DRA is based on stimulated Raman scattering, an inelastic scattering process between photons and optical phonons in which optical power is transferred from shorter to longer wavelengths.
FIG. 1 shows a typical Raman gain profile (Raman gain spectrum). The maximum power transfer occurs between wavelengths separated by 13.3 THz (about 100 nm). Two pumping arrangements can be distinguished, as shown in FIG. 2 and FIG. 3.
FIG. 2 shows counter-directional pumping, where the pump light 112 propagates in opposite direction to the signal light (signal waves) 111 in a transmission fiber 101. In this case, a discrete amplifier 103 and a pump unit 104 are provided in a repeater unit on one side (output side) of the transmission fiber 101 and coupled to the transmission fiber 101 through an optical coupler 102. The pump unit 104 comprises a plurality of pump lasers of different wavelengths.
FIG. 3 shows co-directional pumping, where the pump light 113 propagates in the same direction as the signal light 111. In this case, a pump unit 106 is provided in a repeater unit on the other side (input side) of the transmission fiber 101 and coupled to the transmission fiber 101 through an optical coupler 105.
In state-of-the art systems, counter-propagation is commonly used in order to avoid the risk of pump-signal crosstalk.
Employing a plurality of pumps with different wavelengths and suitable power allows flattening the gain over a wide signal wavelength range as required in broadband WDM transmission systems (see Reference 1).
Reference 1
L. Labrunie et al., “1.6 Terabit/s (160×10.66 Gbit/s) unrepeatered transmission over 321 km using second order pumping distributed Raman amplification”, OAA 2001 PD3.
FIG. 4 shows this principle schematically with a power spectrum for a C/L-band transmission system with four Raman pumps of different wavelengths (first-order multiple-wavelength Raman pumping). Pump light of frequencies f1, f2, f3, and f4 (f1>f2>f3>f4) amplifies L-band and C-band signal waves with an appropriate gain over the signal wavelength range.
Since the Raman pumping efficiency is polarization sensitive it is necessary to use depolarized pump light in order to suppress polarization dependent gain. Depolarization can be achieved by multiplexing two waves with orthogonal polarization of the same or of slightly different frequencies fp1, fp2 given by fp1=fp−δfp, fp2=fp+δfp, where δfp is up to 0.35 THz. Later on, the term “pump” will be used for such pairs of multiplexed waves with slightly different frequencies and orthogonal polarization. As frequency of a depolarized pump the center frequency fp is used.
The group velocity vg, i.e. the speed at which an optical pulse propagates through a fiber, is wavelength dependent. This phenomenon is known as group velocity dispersion or chromatic dispersion. FIG. 5 shows schematically the wavelength dependence of the group velocity of a standard single mode fiber (SMF). In the normal dispersion regime, the higher frequency components travel slower than the lower frequency components (λ<λd, β2>0). Group velocity dispersion parameter β2 is written as follows by using wavelength λ (ω=2π/λ) and dispersion parameter D.
                              β          2                =                                            ⅆ                              ⅆ                ω                                      ⁢                          (                              1                                  v                  g                                            )                                =                                    -                                                λ                  2                                                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  c                                                      ⁢            D                                              (        1        )            
Chromatic dispersion causes pulse broadening because the individual spectral components of the pulse propagate at different speeds.
Recently, two new technologies have been introduced to further improve the performance of Raman amplified transmission systems: second- and third-order Raman pumping and Raman pump modulation.
Second order Raman pumping uses a second order pumps to amplify first order pumps along the transmission fiber. This makes the gain experienced by the signals more uniform along the fiber, which improves the noise figure. It has first been proposed with the first-order pump counter-propagating and the second-order pump co-propagating to the signals (see Reference 2).
Reference 2
K. Rotwitt et al., “Transparent 80 km bi-directionally pumped distributed Raman amplifier with second order pumping”, European conference on optical communications 1999, vol. II, pp. 144-145.
Later, second-order Raman pumping with both the first- and second-order pumps counter-propagating to the signals has been demonstrated (see References 1 and 3).
Reference 3
Y. Hadjar et al., “Quantitative analysis of second order distributed Raman amplification”, OFC 2002 ThB pp. 381-382.
Third-order Raman pumping has also been demonstrated (see Reference 4).
Reference 4
S. B. Papernyi et al., “Third-order cascaded Raman amplification”, OFC 2002 postdeadline papers, FB4.
Fludger et al. proposed modulation and temporal separation of multi-wavelength Raman pumps as a means to suppress stimulated Raman scattering and four-wave mixing (FWM) among pumps (see Reference 5). Both effects degrade the performance of broadband WDM transmission systems.
Reference 5
C. R. S. Fludger et al., “Novel ultra-broadband high performance distributed Raman amplifier employing pump modulation”, OFC 2002 WB4.
As Fludger et al. have pointed out, the modulation frequency of the Raman pump light should be in the order of a few 10 MHz to several 100 MHz. If the modulation frequency is too low, modulation transfer from the pumps to the signals occurs. On the other hand, if it is too high, the pump pulses are dispersed.
Since the Raman effect is practically instantaneous when compared to signal bit rates, modulation of the pump intensity will cause variations in the gain experienced by the signals. However, in counter-directional pumping scheme, the gain is averaged over the effective length of the transmission fiber such that any pump fluctuations above a few KHz are filtered. Further, modulation transfer from the pumps to the signals is negligible for the modulation frequency greater than a few MHz (see References 5 and 6).
Reference 6
C. R. S. Fludger et al., “Pump to signal RIN transfer in Raman fiber amplifiers”, Journal of lightwave technology, Vol. 19, No. 8, pp. 1140-1148, August 2001.